Cardiac arrhythmias continue to be a leading cause of death and disability. Despite this alarming fact, a true imaging modality for cardiac electrical function has not yet been developed. Standard electrocardiographic techniques attempt to infer the electrical state of the heart from a limited number of recordings on the body surface. This traditional approach is very limited in its ability to provide information on regional electrocardiac activity and to localize bioelectric events in the heart (e.g., arrhythmogenic foci; regions of elevated dispersion of myocardial repolarization). The objective of this project is to continue the development of a novel imaging modality ("Electrocardiographic Imaging", ECGI) for the reconstruction of cardiac electrical activity from potentials measured away from the heart (i.e., on the torso surface and in the blood cavity), and to further our understanding of the relationship between the cardiac excitation and repolarization process and the electrocardiographic potentials that they generate. During the current support period, the investigators have developed the methodology and demonstrated feasibility of the approach i normal hearts, in the absence of structural heart disease. Goals for the next grant period are: (1) to continue the development of ECGI and to evaluate its ability to reconstruct potentials, electrograms, and activation sequences on the surfaces of the heart in the presence of infarction and during reentrant ventricular tachyaarhythmias; (2) To characterize the relationship between regionally elevated dispersion of myocardial repolarization (an arrhythmogenic substrate, e.g., in the long QT syndrome or hypertrophic cardiomyopathy) and its reflection in body surface potentials. To apply and evaluate the ability of ECGI to noninvasively detect, quantitate and reconstruct, from potentials measured on the body surface, regions of high dispersion of myocardial repolarization in the heart; (3) To continue implementation of ECGI in the clinical setting and to test this modality in selected groups of patients that provide an opportunity to validate and evaluate the approach. Health relatedness: Sudden cardiac death remains a major unresolved public health problem (more than 300,000 deaths annually in the United States alone). The investigators propose to continue the development of an electrocardiographic imaging modality for (1) noninvasive identification of patients at high risk of sudden cardiac death before they suffer a catastrophic event, so that preventive measures can be taken; (2) noninvasive diagnosis of cardiac arrhythmias and their mechanism to that specific therapy can be indicated; (3) noninvasive evaluation of the efficacy of therapy (e.g., drugs); (4) localization of foci of arrhythmogenic activity prior to nonpharmacologic intervention (e.g., surgical or catheter ablation).